Application of secondary ion mass spectrometry to the study of carburization of superalloy

Secondary ion analysis was applied to the study of the carburization behavior of a superalloy by using the ion microprobe mass analyzer (IMMA). The corrosion test of Inconel 617 was carried out for 1000 h under an environment of methane-doped impure helium gas at 1000 °C. 20 KeV O ₂ ⁺ and N ₂ ⁺ were used as primary ions. IMMA showed that C, O, Al, Si, Ti and Mo were enriched near the surface. The chemical states of the scale and the precipitates were determined from chemical shifts of X-ray emission spectra. It was confirmed that silicon carbide and molybdenum carbide were coprecipitated in the same areas: in the scale, in the matrix at a depth of 10 μm to about 40 μm below the scale, and at grain boundaries. Aluminum and titanium were found to exist as oxides near the surface. It was shown that carburization was limited near the surface, and the grain boundary precipitation of carbides in the bulk was due to aging at high temperatures. The concentration profiles of silicon and molybdenum in (Si,Mo)-carbide were obtained by using standard carbide samples assuming that the concentration of implanted oxygen atoms of the specimen was equal to that of the standard sample. The ratio, (Si)/(Mo), was increased from about 0.1 in bulk to 2.4 ∼ 3.8 near the surface. This result indicates that silicon is carburized much more preferentially than molybdenum.